Method of making aluminum base alloy extruded product



United States Patent 3,113,052 METHGD OF MAKHNG ALUMENUM BASE ALLOY EXTRUDED PRGDUCT Kenneth H. Schneelr, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa, a corporation of Pennsylvania No Drawing. Filed July 5, 1960, Ser. No. 40,522 5 Claims. (Cl. 143-415) This invention relates to making aluminum base alloy extruded products, and it is especially concerned with extruded products which receive a solution heat treatment to develop a high strength.

The metal working process known as extrusion involves pressing metal stock through a die opening of predetermined configuration to form a shape of indefinite length, and substantially constant cross section. In the direct extrusion process, with which this invention is concerned, the preheated aluminum base alloy stock is placed in a cylinder, usuallylieate'd, having a suitable die \at one end a reeiprocable piston or ram of approximately the same cross sectional dimensions as the bore of the cylinder which moves against the stock to compress it and cause the metal to flow through the die opening. The pressure exerted on the stock during the operation raises the internal temperature of the stock as a result of the internal friction within the metal body. Moreover, the metal flow is not uniform across the cross section of the stock because of friction with the walls of the cylinder and the creation of so-called dead spots adjacent the die opening. While such irregularity of metal flow within the cylinder has no adverse effect upon many extruded products, particularly those of aluminum and those alloys which do not require solution heat treatment, in other cases a banded internal structure is developed, especially toward the rear of the extrusion such that when the extrusion is subsequently solution heat treated the metal adjacent to the extelnal surface undergoes recrystallization and comparatively coarse grains appear. The metal within the central portion does not usually recrystallize. As a result of this difference in grain structure the tensile properties are not uniform, and the average tensile strength values are lower than where; no recrystallization has occurred.

The problem of minimizing recrystallization is more difficult in the case of some alloys than in others. Genenal-ly speaking, extrusions of those alloys which contain on the order of 1 or 2% of added elements are prone to recryst-allize when subjected to a solution heat treatment. in particular, it has been found that alloys of the aluminum-magnesium silieide type show a banded structure in solution heat treated extruded products.

Another factor which affects the quality of an extruded product is the speed of extrusion. Experience has shown that to achieve acceptable surface quality a certain range of speeds must be observed, the range being related to the size of the extrusion and reduction in cross sectional area eiieoted by extrusion. Exceeding the predetermined speed usually causes a rupture of the surfiace and other defects which result in rejection of the product. It has been found that increasing or decreasing the extrusion speed from the normal range can aifect the susceptibility to recrystallization but only at the expense of surf-ace quality.

With these and related problems in view it is an object of this invention to provide a method of making solution heat treated extruded products of certain aluminum base alloys which do not exhibit a banded recrystallized structure.

Another object is to provide a method of making solution heat treated extrusions of an aluminum base alloy which have substantially uniform strength from front to rear of [the extruded article.

A further object is to provide a method of producing extensions of aluminum base alloys under conventional pressure and speed conditions without developing a recrystallized coairse grain structure in the solution heat treated product.

These and other objects and advantages or" the invention will be apparent from the following description and examples.

My invention is predicated upon the discovery that extruded and solution heat treated products can be made from the aluminumnragnesium silicide type of alloy which are substantially free fmom a recrystallized coarse grain structure by a proper preheating of the ingot from which the extrusion is produced and by subjecting the extruded product to a sub-normal solution heat treating temperature. By means of these combined treatments extruded products have been consistently obtained which have a uniform unrecrystallized grain structure throughout the product and a concomitant uniformity in strength. The level of strength developed by the combined treatments meets the strength requirements for alloys treated in conventional manner. These results are achieved without departing from the usual extrusion practices.

The aluminuinmagnesium silicide type of alloy referred [to above consists of those alloys which contain about 0.75 to 2% of the intermetallic compound magnesium silicide, Mg si, as the primary strengthening component. The alloy may contain an excess of magnesium or silicon, usually not more than 10% over the amount required to form the silicide. In terms of proportion by weight this means employing from 0.43 to 1.40% magnesium and 0.24 to 0.80% silicon. In addition to magnesium and silicon the alloy may contain up to 0.5% copper and preferably not less than 0.05%. At least one of the grain refining elements boron, titanium, cliromium, manganese, molybdenum, tungsten and zirconium may also be included in amounts of 0.01 to 0.30% each, with the exception of boron which should be used in the proportion of 0.001 to 0.10% by weight. The total amount of the grain refining elements should not exceed 0.75%. From 0.0005 to 0.02% beryllium may also be included to improve the resistance to oxidation. The usual impurities may be present and the iron impurity can be tolerated in amounts up to 0.75%.

The stock normally used for the production of extruded aluminum base alloy articles is in the form of ingots or cast billets. The severe working of the metal incident to extrusion converts the cast structure of the ingot into a wrought structure. The ingots may be produced by any of the well known casting processes, the continuous or semi-continuous method being the one most commonly used at present. Because the ingot is cast, there is a certain amount of inhomogeneity in the structure and a thermal treatment is necessary to effect soluion or" any soluble elements and alloy constituents. Such a homogenizing treatment has the further advantage of improving the working characteristics of the ingot. For the purpose of this invention the preliminary thermal treatment of the ingot, also referred to as a preheat, should consist of heating the ingots at 800 to 1050 1 1, depending on the alloy, for a lsufiicient length of time, generally 3 to 20 hours, to cause substantially complete solution of the soluble elements and constituents. Usually a longer period of time is required if the temperature used is in the lower portion of the permissible temperature range.

Following the foregoing thermal treatment the ingots are cooled to room temperature at a retarded rate whereby precipitation of at least a portion of the soluble precipitation hardening elements or alloy constituents is induced. The precipitation in this case is uniformly distributed and does not resemble the coarse segregation which occurs in the 'as-cast product. To develop the desired form and degree of precipitation the ingot is preferably cooled from the preheating temperature to between 700 and 750 F. and held within this temperature range for a period of 2 to 10 hours and then cooled to room temperature.

The thus cooled ingots are reheated to the extrusion temperature, as quickly as possible, preferably by electrical induction means. The heating should be accomplished in a period of less than hour and thus avoid re-solution of any substantial portion of the precipitated constituent. The temperature range for extrusion should be 800 to 850 F. The ingots are extruded under the conventional conditions of pressure and speed, for example, at a rate of 5 to 150 feet per minute.

The extruded products must be subjected to a solution heat treatment as the first step in the process to develop thedesired strength and hardness. However, instead of heating the aluminum-magnesium silicide alloy products to the usual temperature of 970 F., they are only to be heated to 900 to 925 F. The solution heat treating ranges required for the present process are definitely sub-normal with respect to the conventional practice. The treatment must extend over a sufiicient lengthof time to cause substantially complete solution of the previously precipitated alloy constituents. Generally, a period of from 5 minutes to 5 hours at temperature is adequate. It has been found to be necessary, however, to employ sub-normal heat treating temperatures as the last step in the process in order to avoid the occurrence of the undesired banded structure, that is, to prevent the recrystallization of metal adjacent the surface of the extruded product, especially in the rear portion of the product.

Following the solution heat treatment the products are quenched in the usual manner, preferably by means of a Water spray or by immersion in a water bath. If the extruded section is not too thick, an air blast quench may be sufficient.

Although the solution heat treated products may be used in that condition it is usually desirable to develop a high level of strength and hardness by subjecting the products to a precipitation hardening treatment. This is usually accomplished by heating them to a temperature between 300 and 450 F. for a period ranging from 1 to 24 hours, depending on the particular alloy and size of the extrusion. This treatment :does not alter the grain structure nor does it induce recrystallization and thus create a zone of coarse grains.

'In referring to extrusions it is to be understood that they can be of either the solid or hollow type and have any desired configuration.

As an example of the improvement obtained through use of my process reference is made to the production of a hollow extrusion having an out-side perimeter of 25 inches :and a wall thickness varying between 0.6 and 1.0 inch. The alloy had a nominal composition of 1.0% magnesium, 0.6% silicon, 0.25% copper, 0.25% chromium and balance aluminum. The alloy was cast in the form of a hollow ingot having an outside diameter of 25 inches by the conventional semicontinuous direct chill process. The ingots were cut into lengths of 26 inches. One portion of these sections was preheated at 995 to 1025 F. for 12 hours and extruded under customary pressure at a speed of 5 feet per minute at a nominal temperature of 800 The extrusions were given a solution heat treatment at 965 F. for /2 hour, quenched and precipitation hardened by heating to 350 F. for 8 hours, The extruded product had a banded structure of progressively increasing thickness over the rear half of the article. The average tensile strength of the extrusions in the banded section was 43,900 p.s.i., the yield strength 41,800 p.s.i. and the elongation in 2 inches of 16.5%. The minimum values obtained were 34,000 p.s.i. for tensile strength, 32,000 p.s.i. for yield strength and 14% elongation.

A second portion of the ingots was preheated 6 hours at 975 to 1000 F., cooled to 730 F. and soaked at that temperature for 4 hours after which the sections were air cooled to room temperature. The ingot sections were reheated in an induction heater to 800 to 850 F. within a period of 30 minutes and extruded under the same conditions as in the preceding example. The extruded products were given a solution heat treatment at 915 to 925 F. for aperiod of /2 hour, quenched and precipitation harden-ed as in the same manner as above. No banded structure was found in the extruded product and it had an average tensile strength of 45,900 p.s.i., a yield strength of 41,300 p.s.i. and an elongation of 17.5%. The minimum values obtained were 40,000 p.s.i. for tensile strength, 36,000 p.s.i. for yield strength and 16% elongation. While the average values indicate a definite improvement in tensile strength and elongation over those obtained with conventional treatment, the minimum values offer a better comparison. This indicates more clearly the high level of properties attained through the practice of my invention.

Having thus described my invention and certain embodiments thereof, I claim:

1. The method of producing solution heat treated extrusions of an aluminum base alloy composed essentially of aluminum, 0.43 to 1.40% magnesium and 0.24 to 0.80% silicon and in such proportions as to form from 0.75 to 2% magnesium silicide, said heat treated extrusion exhibiting substantially no recrystallized grain structure, said method comprising providing an ingot of the alloy, preheating said ingot at 800 to 1050 F. for 3 to 20 hours, cooling said ingot to room temperature at a retarded rate whereby precipitation of at least a portion of the dissolved constituents is induced, rapidly reheating said ingot to 800 to 850 F extruding said heated ingot, solution heat treating said extruded product at 900 to 925 F., and quenching the product.

{2. The method according to claim 1 wherein the extruded and solution heat treated product is precipitation hardened by heating it to a temperature between 300 and 450 F. for a period of 1 to 24 hours.

3.- The method according to claim 1 wherein the alloy also contains 0.05 to 0.5% copper.

4. The method according to claim 1 wherein the alloy also contains at least one grain refining element of the group consisting of boron, titanium, chromium, manganese, molybdenum, tungsten and zirconium in amounts of 0.01 to 0.30% each except boron in amounts of 0.001 to 0.10%, the total amount of said grain refining elements not exceeding 0.75%.

5. The method according to claim 1 wherein the alloy also contains from 0.0005 to 0.02% beryllium.

References Cited in the file of this patent UNITED STATES PATENTS 2,249,353 Fritzlen July 15, 1941 2,381,714 Beck Aug. 7, 1945 2,658,845 Barker Nov. 10, 1953.

OTHER REFERENCES Alloy Digest, published by Engineering Alloy Digest Inc. Alloys listed under filing code: Al-17, June 1954,, and Al-23, February 1955, relied upon. 

1. THE METHOD OF PRODUCING SOLUTION HEAT TREATED EXTRUSIONS OF AN ALUMINUM BASE ALLOY COMPOSED ESSENTIALLY OF ALUMINUM, 0.43 TO 1.40% MAGNESIUM AND 0.24 TO 0.80% SILICON AND IN SUCH PROPORTIONS AS TO FORM FROM 0.75 TO 2% MAGNESIUM SILICIDE, SAID HEAT TREATED EXTRUSION EXHIBITING SUBSTANTIALLY NO RECRYSTALLIZED GRAIN STRUCTURE, SAID METHOD COMPRISING PROVIDING AN INGOT OF THE ALLOY, PREHEATING SAID INGOT AT 800 TO 1050*F. FOR 3 TO 20 HOURS, COOLING SAID INGOT TO ROOM TEMPERATURE AT A RETARDED RATE WHEREBY PRECIPITATION OF AT LEAST A PORTION OF THE DISSOLVED CONSTITUENTS IS INDUCED, RAPIDLY REHEATING SAID INGOT TO 800 TO 850*F., EXTRUDING SAID HEATED INGOT, SOLUTION HEAT TREATING SAID EXTRUDED PRODUCT AT 900 TO 925*F., AND QUENCHING THE PRODUCT. 